1. Field of the Invention
The present invention relates to a multifaceted reflecting mirror comprising a plurality of reflecting mirror elements to be used for illuminating an object with a light beam having a uniform illuminance distribution, and a method for producing the multifaceted reflecting mirror. The present invention also relates to a reflection type illumination system to be used for illuminating an object with a light beam having a short wavelength of not more than 200 nm with a uniform illuminance distribution, and an exposure apparatus provided with the reflection type illumination system.
2. Description of the Related Art
At present, the research and development is actively performed in order to further narrow the line width of the circuit pattern in the production of semiconductor devices such as DRAM and MCP. Various techniques have been developed in order to realize line widths of 0.13 μm (4G DRAM), 0.1 μm (16G DRAM), and 0.07 μm (32G DRAM) as the design rule.
In general, when it is intended to narrow the line width of an exposure pattern by using a semiconductor exposure apparatus, a diffraction phenomenon is caused by the exposure light beam. The occurrence of any unintentional diffraction image and the occurrence of defocus of light collection point, which result from the diffraction phenomenon, become problems. In order to solve the problems, it is recommended that the numerical aperture (N.A.) of the exposure optical system is increased and the exposure wavelength is shortened.
However, if the wavelength of light is shortened, especially if the wavelength is not more than 200 nm, then it is difficult to get an optical material which has a low absorbance and which is excellent in workability. For this reason, a reflection type projection optical system has been developed in place of the conventional transmission type projection optical system. For example, the following method is known. That is, a circular arc-shaped optical field (area capable of being used as an exposure area) is realized for the soft X-ray by combining a plurality of reflecting mirrors. A comparted area (chip-forming area) on a substrate is subjected to the exposure by moving a mask and a wafer mutually synchronously at a relative velocity corresponding to the ratio of projection reduction magnification of a projection optical system. Koichiro Hoh and Hiroshi Tanino, “Feasibility Study on the Extreme UV/Soft X-ray Projection-type Lithography”, Bulletin of the Electron technical Laboratory Vol. 49, No. 12, pp. 983–990, 1985.
for the semiconductor production based on the use of an exposure apparatus, it is demanded to improve the throughput as well as narrow the line width. Factors concerning the throughput include, for example, the light emission intensity of the light source, the efficiency of the illumination system, the reflectance of the reflecting mirror, and the sensitivity of the photosensitive material (photoresist) on the wafer. At present, those used as the light source include the ArF laser and the F2 laser, as well as the synchrotron radiation beam and the laser plasma beam as the light source for the light beam having a shorter wavelength. A multilayered film-based reflecting mirror, which is used as a reflecting mirror to reflect the light beam as described above, is developed in order to maintain a high reflectance. For details of the multilayered film-based reflecting mirror, reference may be made to the literature written by Koichiro Hoh et al. described above; Andrew M. Hawryluk et al., “Soft x-ray beam splitters and highly dispersive multi-layer mirrors for use as soft x-ray laser cavity component”, SPIE Vol. 688, Multi-layer Structure and Laboratory X-ray Laser Research (1986), pp. 81–90; and Japanese Patent Application Laid-open No. 63-312640.
The illumination optical system, which is used for the exposure apparatus, is also actively developed. In order to illuminate a master pattern (mask) uniformly without any unevenness, an illumination optical system, which successfully illuminate the master pattern uniformly irrelevant to the light amount distribution of the light source, has been developed. The uniform illumination performance and the aperture performance are required for the illumination optical system. For example, Japanese Patent Application Laid-open No. 60-232552 discloses a technique to generate a rectangular illumination area. The exposure apparatus is provided with a projection optical system for projecting the master pattern onto the wafer. However, the field of the projection optical system is usually circular arc-shaped. That is, the efficiency of use of light is low, because the shape of the field of the projection optical system is different from the shape of the field of the illumination optical system. This fact has exerted harmful influences on the exposure time and the throughput.
Nowadays, Japanese Patent Application Laid-open No. 11-312638 discloses a method for solving the foregoing problem, i.e., a method for setting an illumination field in conformity with a shape of a field of a projection optical system to improve the efficiency of use of light thereby. When this method is used, it is possible to realize an illuminating apparatus and an exposure apparatus provided with the same in which the illumination efficiency is remarkably higher than that of the conventional illuminating apparatus, it is possible to achieve a higher throughput, and it is possible to perform uniform illumination such as the Koehler illumination.
The technique, which is disclosed in Japanese Patent Application Laid-open No. 11-312638, will be explained with reference to FIG. 18. The light beam, which is radiated from a light source 201 of a projection exposure apparatus shown in FIG. 18, passes along a multifaceted reflecting mirror 202, a condenser optical element 203, and a reflecting mirror 204, and it illuminates a mask 205 which is held on a mask stage 205s. A pattern, with which a wafer 207 held on a wafer stage 207s is exposed, is formed as a reflector image on the mask 205. A reflected light beam (reflection pattern) from the illuminated mask 205 is projected onto the wafer 207 via a reflection type projection optical apparatus 206 comprising reflecting mirrors 206a, 206b, 206c, 206d. In this procedure, the optical field of the projection optical apparatus 206 is not so wide as to successfully cover the entire comparted area corresponding to the device chip on the wafer. Therefore, the pattern of the entire chip is formed on the wafer by performing the continuous exposure while synchronizing the mask 205 and the wafer 207 and relatively moving (scanning) them. Therefore, the exposure apparatus is provided with unillustrated laser interferometers for managing the positions of the mask stage and the wafer stage, and a mask stage controller 208 and a wafer stage controller 209 for controlling the driving of the stages. The relative movement of the mask and the wafer is controlled by them. Reference may be made to the literature written by Koichiro Hoh et al. described above for such a scanning type projection exposure system. In this arrangement, the multifaceted reflecting mirror 202 is constructed by arranging one type of or a plurality of types of minute reflecting mirror elements (reflecting surfaces) in accordance with a definite repeating pattern. It is necessary that the outer shape of the reflecting mirror element is similar to the shape of the optical field of the projection optical apparatus. Accordingly, a large number of point light source images I are formed in an approximately circular form at Position P2, and they form a necessary illumination field by the aid of the condenser optical element 203. When the technique as described above is used, then the area on the mask to be illuminated can be illuminated uniformly without any waste, and it is possible to shorten the exposure time. Thus, a semiconductor exposure apparatus having a high throughput is realized.
The present inventors have disclosed a method for producing a multifaceted reflecting mirror of the type as described above in Japanese Patent Application Laid-open No. 2000-162415. In this patent document, in order to form three types of different reflecting mirror elements A, B, C, circular arc-shaped bands (circular arc-shaped bands including circles having an average radius of Zh), which are parallel to the YZ plane, are cut out from a spherical surface having a radius of curvature R with an electric discharge wire at predetermined positions (pattern positions of A, B, C shown in FIGS. 4(a) to 4(c)) as shown in FIGS. 4(a) to 4(c) respectively. The reflecting mirror elements A, B, C, which have been cut out, are arranged in accordance with a predetermined pattern as shown in FIG. 6 to produce the multifaceted reflecting mirror thereby. However, in this patent document, there is no description about the way to secure the reflecting mirror elements on a base plate. In order to enhance the uniformity of the light beam for illuminating the mask therewith, it is necessary that the reflecting mirror elements are installed to the base plate with a high degree of positional accuracy.
A first object of the present invention is to provide a multifaceted reflecting mirror which comprises a plurality of reflecting mirror elements installed onto a base plate with a high degree of positional accuracy. A second object of the present invention is to provide a multifaceted reflecting mirror which is preferably irradiated with a light beam having an extremely short wavelength such as the soft X-ray. A third object of the present invention is to provide an exposure apparatus and an illumination optical system provided with the multifaceted reflecting mirror as described above.